Increasingly strict emission limits are being imposed by regulatory agencies in the United States of America and several other industrialized countries on certain emissions (such as oxides of nitrogen and carbon monoxide) from gas turbine engines. This has resulted in the development of low emission combustion systems. One of the approaches to reducing these emissions utilizes the lean premix combustion concept. In this approach the fuel and air are uniformly premixed before they enter the combustion zone (primary zone) of a combustor and the fuel/air ratio is controlled so that there is a relative excess of air as compared to the stoichiometric fuel/air ratio. Oxides of nitrogen, carbon monoxide, and the like, hereinafter called emissions, from such a combustion system are primarily dependent upon the combustion air inlet temperature, fuel inlet temperature, fuel type, and the fuel/air ratio. It is, therefore, possible to control these emissions by controlling the fuel/air ratio of the combustion zone, the other variables being primarily dependent variables.
The optimum method of controlling emissions would be to control the fuel/air ratio of the combustion zone in response to the measured emissions and using an emissions signal in a feedback control loop to control the fuel/air ratio of the combustion zone. This however, is not practical using state of the art emission analyzers due to a slow response time, poor reliability, poor durability, and problems associated with zero drift and span drift requiring frequent calibration.
Since the emissions are principally dependent on the temperature of the combustion zone gases, it is possible to accurately control the emissions by controlling the temperature of the primary zone gases during engine operation. This requires that a signal proportional to the primary zone temperature be generated so that it can be used in a feedback control loop to regulate the primary zone temperature (through control of the parameters responsible for primary zone temperature such as fuel/air ratio, combustion air inlet temperature, relative humidity, and fuel composition).
Direct generation of a primary zone temperature signal using commonly available devices such as thermocouples, of any type, immersed in the combustion zone is highly unreliable due to their relatively short life at these high temperatures. Radiation pyrometer type sensing devices require optical access to the combustion zone gases and proper filtration of the optical signal in order to eliminate the radiation from the hot combustor surfaces and the hot carbon particles in the hydrocarbon fuel flames. Additionally, the emissitivity of the combustion zone gases must be either directly measured or calculated from the known radiative properties of the combustion gases so that the temperature of the gases can be calculated to generate a signal proportional to the temperature of the combustion zone gases. Due to the errors in the calculation or measurement of the emissivity and the susceptibility of optical lenses or windows to fouling due to the lack of a continuous supply of high purity air, this approach also becomes highly impractical.